The various recent attacks all took place where law-abiding adults could not legally carry handguns for self-defense and largely had to wait for armed police to arrive, but waiting will get you killed:
A lot of people can be shot in the time it takes police to arrive after a mass public shooting begins. The norm seems to be between three and eight minutes.
In every single active shooter attack where a good, armed person was present when the attack began and acted aggressively to stop the killer, the body count was less than 10, single-digits. Every time. 100%. Having a good, armed person present who will act to stop the killer, whether it’s a cop, armed security guard, or armed citizen, ends the attack in the first 1–2 minutes. This prevents the killer from having the time needed to amass a high body count. Waiting on police to arrive and stop the killing gives the killer plenty of time to shoot a lot of people.